A known system of this type for which the link is formed by at least two digital channels is described in the article by J. Y. Auclair et al. entitled "DCN212: equipement de securisation par doublement du conduit numerique a 2 Mbits/s", published in the French journal COMMUTATION & TRANSMISSION, no. 2, 1991, pages 77 to 85. This doubling has for its object to obtain the fewest possible errors. This doubling is effected in two intermediate equipments assigned each to user equipments. In this known system the information is organized in frames and for a certain number of frames or blocks a check code is determined, for example, a CRC (Cyclic Redundancy check) code. Management information is inserted into frames coming from the user equipment. This is effected already in the intermediate equipment assigned to the user equipment on the transmitting side. Before these frames are sent to the second intermediate equipment, another CRC code is calculated, so that the receiving equipment the information relating to blocks poorly received in the first intermediate equipment is lost. This known system cannot indicate the total quality of the link. More and more often the users require to have an indication of the total quality of their link.